Improve the speed of grn_str_charlen_utf8
Improve the speed of grn_str_charlen_utf8

Use GRN_BIT_SCAN_REV to remove the first for-loop.
Simplify the second for-loop.

naoa/groonga,cosmo0920/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,kenhys/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,redfigure/groonga,hiroyuki-sato/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,groonga/groonga,hiroyuki-sato/groonga,redfigure/groonga,myokoym/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,myokoym/groonga,komainu8/groonga,redfigure/groonga,kenhys/groonga,redfigure/groonga,myokoym/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,myokoym/groonga,groonga/groonga,hiroyuki-sato/groonga,groonga/groonga,hiroyuki-sato/groonga,myokoym/groonga,redfigure/groonga,myokoym/groonga,hiroyuki-sato/groonga,groonga/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,redfigure/groonga,naoa/groonga,cosmo0920/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,myokoym/groonga,redfigure/groonga,hiroyuki-sato/groonga,groonga/groonga,myokoym/groonga,naoa/groonga,naoa/groonga,kenhys/groonga,cosmo0920/groonga,cosmo0920/groonga